Induction heating susceptor and method for producing same



1967 w. c. BEASLEY ET INDUCTION HEATING SUSCEPT AND METHOD FOR PRODUCING SAME Filed Sept. 17, 1962 r0 INVENTORS WILLIAM c. BEASLEY 30ml H. BRA, NAN

/ ATTO EV Patented Jan. 3, 1967 3,295,559 INDUCTEGN HEATING SUSCEPTQR AND METHOD FQR PRUDUCENG SAME William C. Beasley and John H. Brannan, Lawrenceburg,

Tenn., assignors to Union Carbide Corporation, a corporation of New York Filed Sept. 17, 1962, Ser. No. 223,921 4 Claims. (Cl. 138141) This invention relates to an improved induction heating susceptor, and more particularly, to a susceptor which is fabricated from a lamination of carbonaceous cloth.

In hot pressing and high temperature coating operations, a cylindrical shell which is commonly referred to as a susceptor is often employed. A susceptor to be acceptable must be made of a material having an electrical resistance of such magnitude that the susceptor will efliciently transform electrical energy into heat when the susceptor is placed in an electrical field. In addition, the susceptor must be fabricated from a material which is capable of withstanding numerous cycles from room temperature to graphitizing temperatures of the order of 2500 C. and above without impairment of the electrical properties. Other properties such as high mechanical strength with a low density, and good mechanical resilience are, of course, also desirable, especially when the susceptor doubles as a retaining wall for high temperature installations.

The type of susceptor which is presently available consists of a thin walled cylindrical shell which has been machined from a block of conventional monolithic graphite. These susceptors have proven acceptable in practice with respect to electrical resistance, but unfortunately they occasionally crack after two or three temperature cycles from room temperature to graphitizing temperature.

Accordingly, the principal object of the invention is to provide a thin walled cylindrical susceptor which has been fabricated from a material which imparts to the susceptor the characteristics of superior thermal shock resistance and temperature stability.

A concurrent object of the invention is to provide a process for fabricating such susceptors.

Broadly stated, the objects of the invention are accomplished by fabricating a susceptor from a lamination of carbonaceous cloth.

The susceptor of the invention and the process for making it will be more clearly understood by reference to the drawing wherein:

FIG. 1 is an end view which illustrates one method for shaping a susceptor which embodies the principles of the invention, and

FIG. 2 is an end view which illustrates a second method for shaping a susceptor which embodies the principles of the invention.

Manufactured carbon and graphite in a flexible textile form has recently become available.

United States Patent 3,011,981, issued December 5, 1961 to W. T. Soltes discloses a method for manufacturing textile carbon from fibrous and substantially pure cellulosic materials, such as strands, skeins, ropes, cloth, fabrics and batting pads. The textile carbon product is reported to be electrically conductive while retaining the flexibility and other physical characteristics of the textile starting material.

Electrically conductive graphite in a flexible fiber and fabric form is reported in Metal Progress, May 1959, pp. -116, and is commercially available in any textile form such as yarns, braids, felts, and woven or knit fabrics, such as cloth.

Laminations made up of a plurality of sheets of such carbon or graphite cloth may be prepared by spraying, dipping or painting the individual sheets of the carbonaceous cloth with a suitable thermosetting or thermoplastic carbonizable binder or cement, stacking the sheets one on top of another, shaping the stacked sheets into the desired shape, treating the binder as required while maintaining the sheets in the desired shape in order to form a self-supporting article, and finally baking and graphitizing the resultant article in an inert or reducing atmosphere.

The preferred binder for use in fabricating the laminated susceptors of the invention is that which deposits the highest amount of carbonizable coke. Among suitable binders for use in the practice of the invention are thermosetting phenolic resins and epoxies and thermoplastic soft and medium pitches. The exact temperature and pressure which is employed in the initial article forming stage depends, of course, on the particular carbonizable binder which is employed and they are readily determinable by those skilled in the art.

If thermosetting binder is employed, a self-supporting shaped article is formed by curing the binder at the appropriate thermosetting temperature under pressure while maintaining the stacked sheets in the desired configuration. When a thermoplastic pitch binder is employed, the forming step depends on whether the pitch is soft or medium pitch. If a soft pitch is employed, the pitch may be dissolved in a solvent or melted and applied to the sheets of cloth, and a self-supporting article may be fabricated simply by the application of pressure. If a medium pitch is employed in a similar manner, it will be necessary to apply heat simultaneously with the forming pressure, after which the resultant article should be cooled in order to allow the pitch to solidify.

Referring now to the drawing, when appropriate, and particularly FIG. 1, a thin walled cylindrical laminated carbonaceous cloth susceptor is initially .shaped by applying, by any suitable means, a suit-able carbonizable binder to a plurality of sheets 10 of carbonaceous cloth, and subsequently wrapping the plurality of sheets 10 around a rigid cylindrical form 12 of such a diameter that the sheets 10 form a continuous cylindrical multi-layer member of uniform thickness. Means for applying the shaping pressure is provided by placing a thin cover 14 suitably of sheet metal, around the plurality of carbonaceous cloth sheets 10, and providing tightening bands 16 around the cover 14. The desired shaping pressure may then be applied uniformly to the plurality of sheets It by tightening up on the bands 16 a sufficient amount.

In the embodiment illustrated in FIG. 2, a thin walled cylindrical laminated carbonaceous cloth susceptor is initially shaped by disposing a plurality of binder treated carbonaceous sheets 10 within a rigid cylindrical form 18.

The diameter of the cylindrical form 18 is such that the sheets form a continuous cylindrical multi-layer member of uniform thickness. Means for applying the shaping pressure is provided by placing a thin expandable liner 20, suitably of sheet metal, against the exposed surface of the plurality of cloth sheets 10, and disposing within the liner 20, an inflatable liner 22. The desired shaping pressure may then be applied uniformly to the sheets 10 by inflating the inflatable liner 22 an amount suflicient to expand the expandable liner against the sheets 10 with the desired force.

Once the desired shaping pressure is provided by either of the disclosed methods, the pressure is maintained and if the binder is thermosetting it is cured by heating to a temperature between about 95 C. and 130 C. The curing pressure is in the general range of from 125 p.s.i. to 200 p.s.i. The same pressure range is suitable for thermoplastic binders and heat, as required, is applied while the pressure is maintained. If heat is required the shaped article should then be allowed to cool.

After the binder in the plurality of carbonaceous cloth sheets has been treated as required, thereby forming a self-supporting laminated article of carbonaceous cloth, a finished susceptor is provided by packing the laminated article in coke and baking it in an inert atmosphere to a temperature of about 800 C. The coke serves as a protective oxidation shield and also aids in supportingv the article when a thermoplastic binder is employed. The article is then subsequently graphitized in an inert atmosphere to a temperature in excess of about 2800 C.

Specific examples for manufacturing the laminated carbonaceous cloth susceptors of the invention are the following.

Example I A plurality of sheets of carbonaceous cloth, each sheet having a thickness in the range of .024 inch to .027 inch, were dipped in a phenolic resin bath and passed through stainless steel rollers gapped at a distance of .012 inch to .024 inch to insure even distribution of binder. A plurality of such sheets were disposed within the apparatus of FIG. 2, the sheets having such a length that a continuous cylindrical member of uniform thickness was formed. A uniform pressure of about 150 pounds per square inch was applied to the plurality of sheets and the binder was thermoset at a temperature of about 130 C. The formed laminate was then baked in a coke pack in an inert atmosphere at the rate of 10 C. per hour to 600 C., 50 C. per hour to 800 C., and graphitized in an inert atmosphere to a temperature in excess of 2800 C. at a rapid rate.

In Table I below various properties of this susceptor are set forth. For purposes of comparison, the same properties are listed for a prior art susceptor which was machined from a monolithic block of graphite.

The electrical resistivity of the cloth laminate is well within the range necessary for eflicient transformation of electrical to heat energy. A low Youngs Modulus for the laminated susceptor is indicative of superior thermal and mechanical shock resistance. In addition, the higher strength-to-density ratio makes the laminated susceptor even more attractive when weight savings are important.

Example II The process of Example I was followed except that a pressure of 12.5 p.s.i. was uniformly applied by the 4 apparatus of FIG. 1.

ceptor are set forth in Table II below.

Example III The process of Example I was followed except that after graphitization the susceptor was impregnated with a pitch furfural mixture and rebaked to 800 C. Typical properties for this susceptor are also set forth in Table II.

A study of the tables indicates that laminated carbonaceous cloth susceptors because of suitable electrical properties, low Y oungs Modulus, and high mechanical strength to density ratios are superior to conventional graphite susceptors. In addition, it is to be noted that the strength of a laminated susceptor is improved by an increase in shaping pressure. Even higher strengths can be realized by impregnating the susceptor with a pitch furfural mixture and rebaking the susceptor. However, since strength improvement results in a higher Youngs Modulus and therefore lower thermal shock resistance, the actual application for the susceptor would decide whether a sacrifice should be made in strength or thermal shock resistance.

It has been further discovered, that when the carbonaceous cloth which is employed as the starting material for making the susceptors of the invention is graphite cloth, susceptors which are eminently suitable for certain low temperature applications may be provided by subjecting the formed self-supporting laminated article of graphite cloth to a baking operation in an inert atmosphere at a temperature of about 800 C. in order to carbonize the binder. For these applications, it has been determined that a final graphitizing of the binder may be omitted.

We claim:

1. A thin walled cylindrical susceptor of uniform thickness which comprises a continuous lamination of graphite cloth.

2. The article of claim 1 wherein said carbonized binder is graphite.

3. A process for manufacturing a thin walled cylindrical susceptor, said process comprising coating a plurality of sheets of carbonaceous cloth with a carbonizable binder, shaping said sheets into a continuous multi-layer cylindrical article of uniform thickness, applying a pressure in the range of from p.s.i. to 200 p.s.i. uniformly to said article and maintaining said pressure while heating said article to a temperature suflicient to cure said binder, further heating said article in an inert atmosphere to a temperature of about 2800 C. to graphitize said article whereby a graphite bonded, continuous cylindrical susceptor of laminated graphite cloth is provided.

4. A process for manufacturing a thin walled cylindrical susceptor, said process comprising coating a plurality of sheets of graphite cloth with a carbonizable binder, shaping said sheets into a continuous multi-layer cylindrical article of uniform thickness, applying a pressure in the range of from 12.5 p.s.i. to 200 p.s.i. uniformly to said article and maintaining said pressure while heating said article to a temperature sufficient to cure said binder, further heating said article in an inert atmosphere to a temperature of about 800 C. to carbonize said binder Typical properties for this sus- 5 6 whereby a carbon bonded, continuous cylindrical sus- 3,053,775 9/1962 Abbott 25242l ceptor of laminated graphite cloth is provided. 3,174,895 3/1965 Gibson et al 161-259 References Cited by the Examiner OTHER REFERENCES UNITED STATES PATENTS 473,841 4/1892 Hulin. EARL M. BERGERT, Primary Examiner. 2,941,570 6/1960 Plym 156156 3,011,981 12/1961 Soltes. R. I. SMITH, M. L. KATZ, Assistant Examiners.

5 Chemical Engineering, May 4, 1959, page 70.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,295,559 January 3, 1967 William C. Beasley et a1.

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 4, line 51, after "cloth" strike out the period and insert therefor bonded together by a carbonized binder.

Signed and sealed this 3rd day of October 1967.

(SEAL) Attest:

ERNEST W. SWIDER EDWARD J. BRENNER Attesting Officer Commissioner of Patents 

1. A THIN WALLED CYLINDRICAL SUSCEPTOR OF UNIFORM THICKNESS WHICH COMPRISES A CONTINUOUS LAMINATION OF GRAPHITE CLOTH.
 2. THE ARTICLE OF CLAIM 1 WHEREIN SAID CARBONIZED BINDER IS GRAPHITE. 